This invention relates to certain aspects of the design of an instrument having a combined and concurrent capability for X-ray diffraction (XRD) with energy-discrimination, and X-ray fluorescence (XRF).
X-ray diffraction is a technique in which a collimated beam of X-rays is directed at a sample of unknown material. The angles of beams of X-rays diffracted from the sample are measured and the interplanar spacings of crystalline materials are determined using Bragg's law, n.lambda.=2dsin.THETA., where .lambda. is the wavelength of the X-radiation, d is the interplanar spacing within the crystal, and .THETA. is the quarter-angle of the cone of diffracted beams emanating from the sample. The resulting list of d-values (interplanar spacings) and diffracted intensities can be compared to equivalent listings from standards (published in computerized data bases such as the PCPDS powder diffraction file) to provide a definitive identification.
When a primary-beam X-ray photon strikes the sample, a variety of other interactions may take place in addition to simple Bragg diffraction. Photon-specimen interactions in which some energy is lost by the X-ray photon result in secondary signals which contain information about sample atoms. For example, X-ray photons can ionize sample atoms by removing inner shell electrons. The resulting inner shell vacancies are filled by electrons from outer shells of the same sample atom, and the difference in energy between the two orbitals is manifested as either an X-ray photon (secondary X-ray) or an Auger electron. The energies of secondary X-rays are characteristic of the elements from which they are emitted, and the number of X-rays and their energies can be translated into major, minor and trace element abundances. This interaction is called secondary X-ray Fluorescence (XRF), and there are known laboratory devices available for XRF analysis.
Commercially available XRD devices to date have not incorporated an XRF capability because of the many compromises which would have to be made for each device to function efficiently under operating conditions common to both. In the laboratory, XRD and XRF data are commonly used together to identify unknown minerals and compounds, but the data are gathered on two separate, large machines.